PSN-L Message - Subject: Re: VRDT Experience

Subject: Re: VRDT Experience To: psn-l@.............. Date: Friday, September 19, 2008, 4:42 PM In a message dated 2008/09/19, barry_lotz@............. writes: I have been running three sensors with the smt8 type triple feedback. The ~= 30" vertical with woofer coil and LVDT seems to work very well. I oscillate= the LVDT with a 6500hz phase shift oscillator. I have a 10 Hz single pole = passive low pass filter after the maxim dg419 demodulator.=20 Hi Barry, =A0=A0=A0=A0=A0=A0 Am I correct in assuming that your feedback is derived f= rom the 10 Hz limited signal? The output of the demodulator needs to be lim= ited to about 1/10 the drive oscillator frequency. This smooths the respons= e and it needs to be about 10x the final maximum response frequency. =A0=A0=A0=A0=A0=A0 What low pass filter are you using on the final output? I then run it thru a gain of 5 and into the triple feedback. The ~12" leaf = spring=20 vertical uses the same 6500 hz oscillator with a VRDT and STM home built=A0= feedback coil. I have been getting a large very low frequency oscillation = around 0.005 Hz. Looking at the winquake helicorder it doesn't last all day= (maybe 2-10 hours at a time) . I have been trying to see if it's environme= ntal but it doesn't appear to occur at the same time periods each day.Could= this oscillation have anything to do with the feedback?=20 =A0=A0=A0 The only circuit element which has a Tc about this is likely to b= e the integrator. For others using the VRDT is degaussing the center element very important a= nd=20 does this have to be done more than once?=20 =A0=A0=A0=A0=A0=A0 There may be a small remanent magnetism in the Silicon I= ron often used in transformer laminations. To get a really stable output le= vel, the lamination should preferably be demagnetised. This only normally n= eeds to be done once - unless the lamination comes in contact with a fairly= strong magnetic field / a permanent magnet / a DC voltage applied across t= he sensor coil windings. =A0=A0=A0=A0=A0=A0 Regards,

Chris

The output of the demodulator goes thru a single pole passive filter set at 10hz and a 4X amp before going to the feedback coil. The velocity output goes thru a line driver and at the end of the cable at the AD board there is a 6 pole lp filter set at 10 hz. I looked back at STM's circuit and I believe he has a three pole passive filter before the feedback. If I understand you correctly, if I have a 6500 hz excitation signal then the low pass after the demodulator shood be ~650 hz and the final signal should be filtered to ~6.5 hz? On an important note. I have to go back to my electronic notes, but with respect to phase, would there be a phase shift of 60 degrees at the output of my passive filter? Is STM using 3 in a row to get a 180 degree shift and then switching the polarity of the feedback coil? The phase shift oscillator uses this principle to generate the oscillation.

Regards

Barry

--- On Fri, 9/19/08, ChrisAtUpw@....... <ChrisAtUpw@.......> wrote:

From: ChrisAtUpw@....... <ChrisAtUpw@.......>
Subject: Re: VRDT Experience
To: psn-l@..............
Date: Friday, September 19, 2008, 4:42 PM

In a message dated 2008/09/19, barry_lotz@............. writes:

I have been running three sensors with the smt8 type triple feedback. The ~30" vertical with woofer coil and LVDT seems to work very well. I oscillate the LVDT with a 6500hz phase shift oscillator. I have a 10 Hz single pole passive low pass filter after the maxim dg419 demodulator.

Hi Barry,

       Am I correct in assuming that your feedback is derived from the 10 Hz limited signal? The output of the demodulator needs to be limited to about 1/10 the drive oscillator frequency. This smooths the response and it needs to be about 10x the final maximum response frequency.

       What low pass filter are you using on the final output?

I then run it thru a gain of 5 and into the triple feedback. The ~12" leaf spring

vertical uses the same 6500 hz oscillator with a VRDT and STM home built feedback coil. I have been getting a large very low frequency oscillation around 0.005 Hz. Looking at the winquake helicorder it doesn't last all day (maybe 2-10 hours at a time) . I have been trying to see if it's environmental but it doesn't appear to occur at the same time periods each day.Could this oscillation have anything to do with the feedback?

    The only circuit element which has a Tc about this is likely to be the integrator.

For others using the VRDT is degaussing the center element very important and

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